How do you/what script do you write to change a scene/level during gameplay?

Questions & Answers
1

my first level consists on three turrets what script should i write for when all three turrets are destroyed i automatically advance into the next level. Also please note how i should write it ex. if(...), function ..., var ..., etc. and what i should attach my script to ex. Character, turrets, etc.

2

Many ways to do it.

One is to create a script that will manage the turrets. When a turret is destroyed, it updates that script about it. That script then checks the status of the other turrets. If all are destroyed - it advances to the next level.

Technically speaking, simplest way is to have something like

public var arrIsTurretAlive: boolean[]; 

public function TurretDestroyed(nTurretID: int)
{
   arrIsTurretAlive[nTurretID] = false;

   var bWasLiveTurretFound: bool = false;
   for (var nCurrTurret: int = 0 ; nCurrTurret < arrIsTurretAlive.Length ; nCurrTurret++)
   {
      if (arrIsTurretAlive[nCurrTurret])
      {
         bWasLiveTurretFound = true;
         break;
      }
   }

   if (!bWasLiveTurretFound)
      Application.LoadLevel(Application.loadedLevel + 1);
}

And FYI - you won't get answers by asking others to do all the work for you. Read some tutorials. Play around and ask specific questions after trying to do it yourself first.